Display screen or portion thereof with transitional graphical user interface



FIG. 1 is a front view of a first image of a display screen or portion thereof with transitional graphical user interface; and,

FIG. 2 is a front view of a second image thereof.

The images sequentially transition from FIG. 1 to FIG. 2. The process or period in which an image transitions to another image forms no part of the claimed design.

The longer broken lines depict an electronic device and a display screen or portion thereof and form no part of the claimed design. The remaining broken lines, including those showing all text, depict portions of the graphical user interface and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with transitional graphical user interface, as shown and described. 